#nav>li{
	list-style: none;/*没有样式*/
	display: inline-block;/*在一行上面*/
	position: relative;
	width: 200px;
}
#nav>li>ul{
	position:absolute;
	display: none;
}

#nav>li>ul>li{
	list-style: none;/*没有样式*/
}

#nav>li>a:hover+ul{/*相邻的兄弟：+*/
	display: block;
}
